Anomalous birefringence of swollen lamellar phases : blue smectics

The birefringence of a lyotropic lamellar phase is calculated as a function of dilution. It is found to vanish and change sign, provided the natural birefringence of surfactant bilayers is positive. Dispersion is calculated about the point of zero birefringence. These predictions are illustrated with experiments of spectrophotometry : the intensity of light transmitted between crossed polarizers through several lamellar samples vanishes as expected at some particular wavelength. The dependence of the transmitted light on wavelength and cell thickness is consistent with theory.La biréfringence d'une phase lamellaire lyotrope est calculée en fonction de la dilution. Nous montrons qu'elle peut s'annuler à condition que la biréfringence naturelle des bicouches de tensioactif soit positive. La dispersion au voisinage du point de biréfringence nulle est calculée. Ces résultats sont confirmés par des mesures de spectrophotométrie : la transmission des échantillons étudiés entre polariseurs croisés s'annule exactement pour une longueur d'onde particulière. La variation de l'intensité transmise en fonction de la longueur d'onde et de l'épaisseur des échantillons est conforme aux prévisions du modèle

Experimental investigations of a new TGBc mesophase

We report on optical and structural X-ray studies on a smectic C twist grain boundary phase (TGBC) of a pure liquid crystalline material. It is shown that this TGBC phase, which exists over a large range of temperature, is definitely different from previously reported and predicted TGBC phases. The two main experimental features are: (i) the observation in planar geometry of an optical texture exhibiting a square grid pattern, and (ii) the detection of a broad Bragg ring in reciprocal space instead of one (TGBA) or two (TGBC) sharp rings. We suggest a few possible tracks that can be explored to understand the complex structure of this new phase